Method for preventing remanence phenomena from interfering with magnetic field sensing systems and a device for implementation of the method

ABSTRACT

In an exemplary embodiment, one or more flux pickups with ferromagnetic properties, preferably Hall generators provided with antennas, are provided for the detection of a field flux or of a change of field flux proceeding from a field generator. Magnetization pulses (I 2 ) are provided in periodic sequence to the flux pickups (I through VIII), the pulse height and width (t im ) being selected in such manner that the saturation field strength (M s ) of the flux pickups is therewith achieved, so as to produce a maximum residual magnetization (M R ). The flux pickups (I through VIII) are preferably driven by means of pulse-shaped signals (I 1 ), with the magnetization pulses being supplied in the pulse interval (i s ) of these signals synchronous to the pulse repetition rate. The method is particularly employed in gnathography for recording the lower jaw movement of a human being.

BACKGROUND OF THE INVENTION

The invention relates to a method for the elimination of remanencephenomena in reception systems and a device for the implementation ofthe method, in which flux pickups with ferromagnetic properties,preferably Hall generators provided with antennas, are provided for thedetection of a field flux or, respectively, of a change of field fluxproceeding from a field generator.

In reception systems in which, in particular, relatively small fieldchanges are to be detected with the assistance of flux pickups such asHall generators, and in which reproducible measuring results are to beachieved with signals gained from a change of field flux, no disruptiveremanence phenomena having a negative influence on the evaluation of thesignals should occur. Such disruptive remanence phenomena, however,occur due to the not absolutely remanence-free ferromagnetic materialsof the flux pickups (Hall generators and, under certain conditions,antennas coupled thereto) given field changes, for example, given achange of location of the reception system in the earth's magnetic fieldor given approach of a measuring instrument with a further fieldgenerator.

An example of employment can be seen in gnathology, where one wishes todetect and record the motion of the lower jaw of a person with theassistance of a measuring device. The proposed measuring method providesthat a magnetic field generator, for example a permanent magnet, besecured to the lower jaw of a patient and, at a distance from themagnetic field generator, a flux pickup system with Hall generators andantennas be arranged on the head of the patient. The field changesoccurring given a motion of the lower jaw are detected by the pickupsystem in all three planes of motion and are evaluated via an electronicevaluation means. The structure of such a device is described in theGerman patent application P No. 28 42 764.2 or, respectively, P No. 2814 551.9, and in the corresponding U.S. Applications Ser. No. 025,263filed Mar. 29, 1979, and Ser. No. 100,146 filed Dec. 4, 1979 now U.S.Pat. No. 4,303,077 issued Dec. 1, 1981.

In order, in particular, to obtain clear and reproducible measuringresults given such measuring devices, it is necessary that no disruptiveremanence phenomena changing the initial point (zero point) of themeasuring system occur.

Since, however, practically no absolutely remanence-free ferromagneticmaterials exist for the flux pickups (Hall generators and antennas) andthe earth's magnetic field also has a disruptive effect on the measuringoperation, one must strive to eliminate the influence of the remanence.

A conceivable way to eliminate these disruptive remanence phenomenawould be a demagnetization of the flux pickups. Such a way is revealedin the publication "Kuhrt/Lippmann, Hallgeneratoren", Springer-Verlag1968, pages 32, 33. The method for this is to allow a magneticalternating field with an amplitude H>H_(s) to influence theferromagnetic material and to then gradually decrease the amplitude ofthis alternating field. The magnetization thus circles the coordinateorigin in hysteresis loops which become smaller and smaller and finally,given a disappearing alternating field amplitude, achieves the pointH=0, M=0.

No external field can have an influence given this demagnetizationmethod, which involves a relatively great expenditure of time. A precisedemagnetization, thus, is only possible in a field-free space. In orderto avoid the earth's magnetic field, a demagnetization is accordinglyonly possible perpendicular to the earth's magnetic field and without ameasuring magnet. This method, thus, is not suitable for many use cases.

SUMMARY OF THE INVENTION

The object of the present invention is to specify a method and a devicefor implementation of the method which are improved with respectthereto, which functions independently of the existing earth's magneticfield and other, potentially disruptive field influences such as, forexample, a measuring magnet or the like, and with which clear andreproducible measuring results can be achieved.

This object is achieved by means of the invention in that magnetizationpulses are supplied to the flux pickups in periodic sequence, the pulseheight and width being selected in such manner that the saturation fieldstrength of the flux pickups is therewith achieved. The flux pickups arepreferably driven by means of pulse-shaped signals, whereby themagnetization pulse is supplied in the pulse interval of these signalssynchronous to the pulse repetition rate. By means of charging thepickup system with magnetization pulses formed by means of short d.c.pulses of specific pulse magnitude in the pulse intervals, as proposed,it is guaranteed that the output voltage at the flux pickup (Hallgenerator) is subject to uniform initial conditions. Since theintermediate pulse is repeated after each measuring pulse with which theflux pickup is driven, uniform initial conditions are thus alwayscreated. After the end of the pulse, the magnetization returns to thevalue M_(R) which represents the maximum possible remanence of thematerial employed for the flux pickup. This value is independent of anyoutside fields which may potentially occur.

Advantageous further developments and embodiments of the invention arecontained in the subclaims. The method is explained in greater detail onthe basis of the Figures of the accompanying sheets of drawings and adevice for implementing the method is revealed. Other objects, featuresand advantages will be apparent from this detailed disclosure and fromthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic perspective view showing a device for detectingthe motion of the lower jaw of a patient;

FIG. 2 is a basic circuit diagram for the drive, on the one hand, of aHall generator with pulse-shaped signals and, on the other hand, of amagnetization coil with pulses which are chronologically offset withrespect thereto;

FIG. 3 is a pulse diagram showing operation according to the inventivemethod; and

FIG. 4 is a hysteresis loop of a ferromagnetic material.

DETAILED DESCRIPTION

In a diagrammatic presentation, FIG. 1 shows a device for determiningthe location, the attitude, and/or a change of location or attitude of apoint of the lower jaw of a patient. In the Figure, 1 indicates the headof a patient and 2 indicates his lower jaw. A permanent magnet servingas the field generator is referenced with 3, being intra-orally securedin the oral cavity of the patient at a desired location of the lower jawby means of suitable adhesive or bonding agents. The magnetic fieldgenerator 3 consists of two identically dimensioned bar magnets as aremore closely described in the German patent application P No. 27 15 106.The magnetic field generator 3 generates two irregular, nonrotationalsymmetrical magnetic fields M₁, M₂ indicated in the Figure with brokenlines.

A magnetic flux pickup arrangement 4 is situated extra-orally of thepatient's mouth, said arrangement essentially consisting of a frame 5supported on the head of the patient 1 and of a pickup system withpickup blocks 6 and 7 situated to the left and to the right of the lowerjaw. The frame 5 is designed in a known manner as a combined spectacleor head frame and contains a plurality of joints (not referenced ingreater detail) for adapting it to the varying head characteristics ofthe patient. The pickup blocks 6 and 7 are rigidly connected to oneanother by means of a rod 8 connected to the frame 5.

Each of the pickup blocks 6, 7 contains four magnetic flux pickups Ithrough VIII which are mounted in a synthetic housing 9, namely, in suchmanner that they respectively lie parallel to one another. The signalspicked up by the magnetic flux pickups I through VIII to be described ingreater detail below, are supplied via multi-conductor lines 10 to anelectronic evaluation means 11 and are supplied from there to a suitableindicator device 12.

The magnetic flux pickup I is partially illustrated in section in orderto explain the structure of the magnetic flux pickups I through VIII.The magnetic flux pickup contains a lamellar Hall generator 13 servingas the sensor, antenna rods 14 and 15 of mumetal of different lengthslying against the effective surface of said Hall generator at bothsides. When the lower jaw is moved, the field flux detected by the Hallgenerators 13 changes. The signals thereby generated by the Hallgenerators are supplied via a preamplifier (not illustrated in theFigure) arranged in the housing 9 to the electronics unit 11, where thesignals are then processed for a visual display.

An induction coil 16 is wound in a single layer around each magneticflux pickup. The single layer winding of the flux pickup has theadvantage that, on the one hand, a good thermal dissipation is givenand, on the other hand, a better magnetic field is generated for themagnetization. The four coils 16 of each block (6, 7) are connected inseries and are connected in parallel to the four others of the otherblock. As shall be described in greater detail below, the coils 16 aresupplied with periodic rectangular pulses by a pulse generator 19 viaone conductive path of a double line 18. Reference numeral 17 indicatesa further pulse generator which drives the Hall generators 13 withpulse-shaped signals via the other conductive path of the double line18.

On the basis of a block diagram, FIG. 2 shows a control circuit for theHall generators 13 which is particularly advantageous for very smallfield strengths, as well as a circuit diagram for driving the coils 16with pulses.

A.C. signals with the mains frequency of fifty or sixty hertz, e.g. 50Hz are supplied via an A.C. transformer 20, said signals being firstsupplied to a Schmitt trigger 21 for pulse shaping. A monostableflip-flop 22 then supplies drive pulses I₁ in a specific mark-to-spaceratio, said drive pulses I₁ being subsequently supplied via a transistor23 to the Hall generator 13.

The pulse signals attainable by means of the circuit revealed nowprovide information concerning the magnitude and waveshape (envelope) ofthe useful signal which is received from the Hall generators 13. Inorder to convert the pulses into a useful signal, these are firstsupplied via an amplifier 24 to a peak value rectifier 25. The signalsobtained therefrom can then be evaluated and optically displayed withthe assistance of a suitable evaluation (computer) and indicator means11, 12.

The pulses generated by the clock generator 17 and, for example, withthe assistance of the circuit arrangement according to FIG. 2 arereferenced with I₁ in the pulse diagram according to FIG. 3. In the usecase, the pulse width t_(is) amounts to approximately 0.5 ms given apulse period T_(is) of 20 ms. The mark-to-space ratio (pulse width topulse interval) t_(is) /i_(s) is selected in the present case in suchmanner that the allowed effective driving power of the Hall generator isnot exceeded, i.e., the area referenced with F₁ dare not be greater thanthe area referenced with F₁ ' which corresponds to the driving power ofthe Hall generator given direct current (D.C.) excitation. The moreextreme the mark-to-space ratio is, i.e. the shorter the pulses and thelonger the pulse pauses are selected, the higher the maximum amplitudeof the control current can be set and, thus, the more the sensitivity ofthe Hall generator can be increased.

In the pulse interval i_(s), i.e. in the pulse pauses, short D.C. pulsesI₂ are periodically supplied to the coils 16 with the assistance of thepulse generator 19. A circuit diagram for generating these pulses isrevealed in FIG. 2. The drive pulses I₁ for the Hall generators 13 aresupplied to a monostable flip-flop 26, a further monostable flip-flop 26being driven with their trailing pulse edges. The flip-flop 26determines the delay time, referenced in the pulse diagram according toFIG. 3 with t_(V), with respect to the drive pulse I₁ ; the flip-flop 27determines the pulse width with which the transistor referenced in thecircuit diagram according to FIG. 2 with 28 and, thus, the magnetizationcoils 16 are driven. The pulses I₂ are supplied synchronous to the drivepulses I₁, with a frequency of 50 Hz in the present case. The point intime at which the pulses I₂ are supplied can, of and for itself, berandomly selected; it need only lie within the pulse pause i_(s). Pulseheight and pulse width are to be selected great enough that thesaturation field strength for the flux pickups (Hall generator andantenna) is achieved with the magnetization pulses induced by the coils16. The magnetization of the antenna system thereby achieves itssaturation value M_(S), in accord with the illustration of FIG. 4, whichshows a hysteresis curve of a ferromagnetic material. The saturationfield strength of the material provided for the pickups can thus alwaysbe achieved by means of the D.C. pulse I₂ periodically supplied in thepulse interval i_(s). After the end of the pulse, the magnetizationreturns to the value M_(R). This value, since it represents the maximumpossible remanence for the system, is independent of any outside fieldswhich may potentially occur. Since a repetition of the operation ensuesafter each measuring pulse, i.e., every twenty milliseconds (20 ms) inthe exemplary embodiment, it is guaranteed that the output voltage atthe Hall generators 13 is always subject to the same initial conditions.

The inventive method can also be employed given a non-pulsed controlcurrent. Given uniform control current, the magnetization pulses aresupplied in periodic sequence to a desired location, whereby the controlcurrent or the measuring voltage is then blanked out during the durationof the magnetization pulses.

The coils 16 on each flux pickup I through VIII are geometricallyidentical, i.e., they have the same number of winding turns, the samediameters and lengths. If, during the measuring time t_(is), a further,short D.C. pulse I₃ of specific size is connected to all pickups, as isrevealed in the pulse diagram according to FIG. 3, then respectivelyidentical magnetic fields are generated at the Hall generators 16. Withthe assistance of matching amplifiers provided for this purpose, theHall generators can thus be balanced to the same sensitivity. By meansof this feature, a simplified balance and an easier control of thebalance can be achieved. The D.C. pulses I₃ arise by means of parallelconnection of a resistor 30 via a transistor 31 to the transistor 28.The direct (D.C.) current I₃ is not present during a measurement.

In order to avoid the so-called "reversal error" in a field reversal(cf. publication Kuhrt/Lippmann, Pages 143/144), it is proposed in anadvantageous development of the invention for the balance of the deviceor, respectively, for checking the balance, that the coils 16 be loadedwith direct (D.C.) currents of equal magnitude and polarity (I₄ in FIG.3) by means of the parallel connection of a resistor 29 to thetransistor 28. A so-called offset voltage is generated at the amplifieroutputs in this manner--upon condition that these were balanced to"zero" in the field-free state. If one selects the current strength,which corresponds to the field strength of each individual coil, highenough that it is greater than the sum of the field strength of theearth's magnetic field and the magnetic field, at no time will a fieldreversal and, thus, a reversal error occur at the Hall generator.

It will be apparent that many modifications and variations may beeffected without departing from the scope of the novel concepts andteachings of the present invention.

I claim as my invention:
 1. Method for preventing disruptive remanencephenomena in reception systems in which a plurality flux pickups withferromagnetic properties are provided for the detection of a field fluxand of a change of field flux proceeding from a field generator,comprising the steps of:supplying a magnetization pulse to each fluxpickup, said pulse having a characteristic such that the pulse heightand width cause saturation of each flux pickup; measuring the field fluxfrom the field generator with each flux pickup after the magnetizationstep, and during a period when the remanence of each pickup due to saidsaturation is at a maximum.
 2. Method according to claim 1,characterized in that the flux pickups (I through VIII) are Hallgenerators provided with ferromagnetic antennas,the magnetization stepresulting in the saturation of the ferromagnetic antennas of the Hallgenerators; the measuring of the field flux from the field generatorwhich is coupled with each Hall generator, after the saturation of theferromagnetic antennas by the magnetization step, being effected bysupplying a control pulse (I₁) of electric current through each Hallgenerator, with the control pulses (I₁) being supplied periodically toeach Hall generator and having a pulse interval (i_(s)) between thesuccessive control pulses (I₁); and the magnetization step comprisingsupplying each magnetization pulse (I₂) during the pulse interval(i_(s)) between successive control pulses (I₁), and said method furthercomprising:supplying the magnetization pulses (I₂) periodically at apulse repetition rate synchronous to the repetition rate of said controlpulses (I₁).
 3. Method according to claim 2, characterized in that themagnetization pulses (I₂) are supplied with the same pulse repetitionrate as the control pulses (I₁).
 4. Method according to claim 1,characterized in that the magnetization pulses (I₂) are rectangularpulses.
 5. Method according to claim 1, wherein a plurality of the fluxpickups each includes ferromagnetic material with an electric winding(16),said magnetization step comprising supplying the magnetizationpulse as an electric current pulse (I₂) to the electric winding (16) ofeach flux pickup; said method further comprising:continuously supplyinga direct current (I₄) of identical magnitude and polarity to theelectric winding (16) of each flux pickup so as to prevent a fieldreversal at the Hall generators during the field flux measuring step. 6.Method according to claim 1, characterized in that the method isemployed (FIG. 1) for detecting the motion of the lower jaw of a humanbeing.
 7. A device for practicing the method according to claim 6,characterized in that two flux pickup blocks (6, 7) are symmetricallysecured to a frame (5) arranged at a fixed relationship to a patient'shead (1), four respective flux pickups (I through VIII) being arrangedon each of said flux pickup blocks (6, 7).